The onboard TAWS systems on board aircraft are responsible for the prevention of aeronautical accidents in which an aircraft that is still maneuverable crashes. Accidents of this type, known in the technical literature by the acronym CFIT, standing for “Controlled Flight Into Terrain”, in the past constituted a significant percentage of air disasters. They are now mostly avoided, thanks to terrain avoidance maneuvers performed by the crews driven by warnings and alarms originating from TAWS systems, included in which are the GCAS (Ground Collision Avoidance System) and T2CAS (Terrain & Traffic Collision Avoidance System) systems, developed and marketed by Thales.
The TAWS systems use a so-called FLTA (Forward Looking Terrain Avoidance) function which watches, in front of the aircraft, along and below its flight path vertically and laterally, to see if there is a potential risk of collision with the terrain. Their principle is based on monitoring the penetration of the terrain into one or more protection volumes linked to the aircraft based on a modeling of the terrain being flown over and on the warnings and alarms issued each time the terrain penetrates into a protection volume.
The problem posed by the TAWS systems is that the end of a warning or an alarm is an indication of the effectiveness of the avoidance maneuver undertaken, but not of the end of the conflict with the terrain which occurs only when the aircraft can resume a normal flight.
In the absence of “end of conflict with the terrain” signal, the crew of an aircraft waits until it is clearly above a safe altitude to terminate a terrain avoidance maneuver undertaken following a warning or alarm originating from a TAWS system, which unnecessarily prolongs the flight time.
To resolve this problem, the applicant has already proposed, in French Patent Publication No. FR 2.848.661, a TAWS system using, in addition to the protection volumes linked to the aircraft and configured for the detection of the risks of collision with the terrain, an additional protection volume linked to the aircraft, especially configured to detect the moment when the aircraft has the possibility of terminating the avoidance maneuver to fly horizontally or resume the bearing and gradient followed prior to the avoidance maneuver. This additional route resumption protection volume is defined, like the other protection volumes, by its lower and front part which serves as a sensor and must not come into contact or, a fortiori, penetrate into the terrain for there to be the possibility of terminating an avoidance maneuver.
After a successful terrain avoidance maneuver, the crew of the aircraft can return to the path provided in its flight plan because, in practice, when a TAWS system issues a justified alarm, the aircraft is no longer on the path provided in its flight plan. With the end of conflict with the ground signal delivered by the abovementioned TAWS system, the crew has, depending on the configuration adopted for the additional route resumption protection volume, either the possibility of resuming a horizontal flight but with no guarantee of freedom of maneuver in the horizontal plane or information on its lower vertical speed margin, or the possibility of resuming the bearing and the gradient followed prior to the avoidance maneuver, which may not be sufficient to return to the path initially provided when the risk of collision with the ground that has been avoided is the consequence of a lateral path error.